1. Field of the Invention
The present invention relates to a field effect transistor and more particularly to an improved high electron mobility field effect transistor (HEMT) using electrons quantized into 2D.
2. Description of the Related Arts
A high electron mobility field effect transistor (HEMT) which is composed of such compound semiconductors as GaAs, a material having a higher mobility than Si, and uses 2D electron gas (2DEG) layer is known as a transistor suitable for high speed operations such as amplification of high frequency and high speed switching. The HEMT is characterized in that a layer (channel layer) in which electrons travel and a layer (donor layer) through which the electrons are supplied are separated, and the electrons are stored as 2D electron gas in a quantum well created at the hetero-interface of the channel layer and the donor layer. A spacer layer may be interposed between the channel layer and the donor layer to prevent scattering of the electrons.
A structure called a pseudomorphic HEMT using InGaAs which has a higher electron mobility than GaAs for the channel material has been proposed in an effort to improve the performance of the HEMT. Although the electron mobility is surely improved by adding In to GaAs, it is known that the lattice constant is increased and strain is applied to the crystal lattice of the InGaAs layer.
If a layer having a different lattice constant is grown on a substrate material, strain is applied to the crystal growth layer and the bond of atoms is cut by the strain, causing dislocations. It is known that there exists a certain regularity in the direction in which the dislocations are caused and according to "Journal of Crystal Growth 111" (1991) pp. 479-483 for example, it is reported that in a case of crystal growth in the [100] axial direction, dislocations along [011] axis are preferentially generated when the crystal growth layer is subjected to compression strain and when it is subjected to tension strain in contrary, dislocations in the direction along [011] axis are preferentially generated.
When electrons traveling in the channel layer cross the dislocated plane, they are scattered and the mobility of the electrons is degraded. Further, the flow of the electrons crossing the dislocated plane is blocked because the amount of stored electrons is locally reduced at the portion where the dislocated plane existing in the donor layer or spacer layer contacts with the channel layer. In any case, it results in increased resistances due to the aforementioned reasons. If dislocations are produced preferentially in certain crystal orientations in a crystal layer having such strain, a phenomenon in which the resistances vary depending on crystal axes, i.e. a resistance anisotropy, is brought about. Although elements have been formed without paying any attention to this resistance anisotropy in the prior art, there has been a problem that when elements are to be formed on a semiconductor operating layer having such resistance anisotropy, performance of the elements is degraded due to the influence of the resistance anisotropy as the increase of parasitic resistance or the degradation of mobility of electrons by the scattering, if the elements are disposed in a manner flowing electrons along the direction where the resistance is high.
Accordingly, it is an object of the present invention to overcome the aforementioned problems by providing a field effect transistor which is able to suppress the influence of degradation of performance of elements caused by to the resistance anisotropy.